In automotive blower motor installations, several discrete blower speeds are customarily achieved through the use of various resistor elements connected in series with the blower motor via a multi-position speed control switch. In such installations, the resistor elements may be formed in an array on a support structure disposed in the inlet or discharge air path of the blower to dissipate heat generated by the passage of motor current through the resistor elements. The resistor elements may be discrete (wire-wound, for example) or printed in an array on a substrate in the manner of a printed circuit board.
The motor current path is protected by a fuse, and the conventional practice is to design the resistor element assembly to withstand worst case conditions for which the fuse may not provide adequate protection. These conditions include, for example, certain electrical short conditions, stalled blower motor and blocked blower air flow. Alternately, individual fuse or other thermal-responsive elements may be incorporated directly into the resistor array, or the resistor elements may be comprised of PTC materials which increase in resistivity with increasing temperature.